Highly filled paper is an established trend in the paper industry not only due to the savings in the decreased use of fiber, but also due to improved product quality, such as higher opacity and better printability. Calcium carbonate-based fillers are commonly used, because of their superior light scattering properties. A major drawback in the production of highly filled paper, particularly with fillers having high surface area, is the high consumption of sizing agent. Thus, as the content of filler in the paper increases, a larger amount of sizing agent is required in order to obtain corresponding sizing results. Hence, cellulosic suspensions are more difficult to size when the amount of filler increases.
Sizing is primarily performed in order to achieve water repellence in paper or board and reduce edge wicking. It will also affect mechanical properties of paper and board, such as dimensional stability, friction coefficient, pliability and folding endurance. Additionally, sizing may improve printability specifically by controlling ink spreading and adhesion.
The sizing process involves the deposition of hydrophobic substances, commonly referred to as sizing agents, on the fiber surface. Commonly employed sizing agents are non-cellulose-reactive sizing agents, e.g. rosin-based sizing agents, and cellulose-reactive sizing agents, e.g. alkyl ketene dimers (“AKD”) and acid anhydrides such as alkenyl succinic anhydride (“ASA”). It is known, however, that cellulose-reactive sizing agents, i.e. AKD and ASA, undergo hydrolysis that competes with the desired reaction with the fibers. Moreover, sizing losses in the final product can occur due to size inversion or migration, size evaporation, mechanical wear of the product, etc.
Bartz and co-workers have observed that during increased fluidity of AKD wax, some AKD could penetrate and thereafter be trapped in the pore structure of the filler (Bartz, W.; Darroch, M. E.; Kurrle, F. L., “Alkyl ketene dimer sizing efficiency and reversion in calcium carbonate filled papers”, Tappi Journal, Vol. 77, No. 12, 1994). This occurs particularly with the scalenohedral form of PCC, which has the porous rosette structure and high surface area. Voutilainen has shown that fillers with high surface area adsorb AKD even better than fibers (Voutilainen, P., “Competitive Adsorption of Alkyl Ketene Dimer on Pulp Fibers and CaCO3 Fillers”, Proceedings from International Paper and Coating Chemistry Symposium, 1996). The presence of Al- and Si-oxides on the filler surface may additionally adsorb cationic starch contained in the AKD particles. It has also been proposed that a strong interaction, or perhaps even bonding, exists between AKD and calcium carbonate filler. These proposed mechanisms with the filler are naturally undesired, and efforts should be made to minimise these interaction.
To improve sizing efficiency, it is suggested in U.S. Pat. No. 5,514,212 that the surface of the pigment can be modified with an anionic starch-soap complex. Cooked starch from corn or potato is complexed with fatty acid salts and precipitated onto pigment surfaces when mixed with precipitated calcium slurry or papermaking furnish containing high levels of calcium ions.
U.S. Pat. No. 5,972,100 suggests a system consisting of a cellulose-reactive size (such as AKD), a cationic dispersing agent (such as cationic starch or polyamides) and a filler. Aside from improved sizing, the invention allows independent control of both filler loading and sizing separately.
Furthermore, WO 95/13324 refers to calcium carbonate treated with a cellulose derivative such as sodium carboxymethyl cellulose (“CMC”) having a degree of substitution of 0.7. Said treated calcium carbonate is used as filler in alkaline papermaking suspensions whereby the brightness of the paper is increased.
U.S. Pat. No. 3,730,830 discloses a process for making paper, specifically photographic paper, comprising the use of synthetic polymer fibers. Prior to the addition of the synthetic fibers to the fiber suspension, inorganic pigment or carbon is added to a slurry containing carboxymethyl cellulose and the synthetic fibers thereby achieving uniform dispersion of the polymer fibers among the cellulose fibers in the paper stock.
There is still a need for a filler which provides an improved papermaking process and better properties of the paper produced. It would be desirable to provide a filler which renders possible production of highly filled paper showing excellent printing and mechanical properties. It would also be desirable to provide a filler which reduces the sizing demand and hereby results in improved sizing efficiency. It would also be desirable to provide a filler that is compatible with drainage and retention aids, and hereby leads to good drainage, retention and paper machine runnability, It would also be desirable to provide a simple and efficient process for producing a filler showing the above characteristics.